tools: Allow testing with javac on buildbot

By running these scripts with 'ANDROID_COMPILE_WITH_JACK=false',
they will instead use javac/desugar/dx to compile the .java files.

Test: art/tools/buildbot-build.sh && art/tools/run-jdwp-tests.sh --mode=host
Test: art/tools/buildbot-build.sh && art/tools/run-libcore-tests.sh --mode=device
Test: art/tools/buildbot-build.sh && art/tools/run-libcore-tests.sh --mode=host
Bug: 36902714
Bug: 37461882
Change-Id: I118d3ec2a3fa31bf85cdcef63d2772df26e07c5e